1. Field of the Invention
The invention relates to a projection objective for projecting a pattern arranged in the object plane of the projection objective into the image plane of the projection objective with the aid of ultraviolet light of a prescribed operating wavelength.
2. Description of the Related Art
Photolithographic projection objectives have been in use for several decades for producing semiconductor components and other finely structured components. They serve the purpose of projecting patterns of photomasks or reticles, which are also denoted below as masks or reticles, onto an object, coated with a photosensitive layer, with a very high resolution on a reducing scale.
Three developments running in parallel are chiefly used in order to generate ever finer structures of the order of magnitude of 100 nm or below. Firstly, an attempt is being made to enlarge the image-side numerical aperture (NA) of the projection objective beyond the currently achievable values into the region of NA=0.8 or thereabove. Secondly, use is being made of ever shorter operating wavelengths of ultraviolet light, preferably wavelengths of less than 260 nm, for example 248 nm, 193 nm, 157 nm or below. Finally, still other measures are being used to increase resolution, for example, phase-shifting masks and/or oblique illumination. In particular, the use of phase-shifting masks requires non-obscured systems, that is to say systems without shading in the image field. Systems without shadows in the image field are generally to be preferred in microlithography, even if obscured systems with otherwise outstanding optical properties are available (for example, DE 196 39 586 corresponding to U.S. Pat. No. 6,169,627 B1).
Limits to angular loadability are reached, above all, for the lenses near the image, when the aperture is increased significantly above NA=0.85. Larger apertures of about NA=1 or above are to be regarded as impractical, since it has to be assumed that because of total reflection in conjunction with such high apertures edge and coma beams can neither be coupled out from an objective nor be coupled into the photosensitive layer of the substrate.